In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket of a sheath system. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High moisture levels inside a cable sheath system may have a detrimental effect on the transmission characteristics of the cable.
Furthermore, water may enter the cable because of damage to the sheath system which compromises the integrity of the cable. For example, lightning or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to move toward a core of the cable, and, if not controlled, to move longitudinally into splice closures, for example. There are some splice closures available commercially in which the cable jacket is terminated inside the closure. Hence, if water is able to travel longitudinally along the cable, it could enter the splice closure, possibly causing a degradation in transmission.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and into the core. For example, a metallic shield which often times is used to protect a cable against electromagnetic interference is provided with a sealed longitudinal seam. However, because lightning strikes may cause holes in the metallic shield, it is not uncommon to include additional provisions for preventing the movement of water longitudinally within the cable.
Filling materials have been used to fill cable cores and atactic or flooding materials have been used to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of a filling material causes housekeeping problems, inhibits manufacturing line speeds because of the need to fill carefully interstices of the core and presents problems for field personnel during splicing operations, for example, it continues to be used to prevent entry of the water into the core. In other arrangements, an atactic material, for example, may be used to flood the outer surface of a metallic shield.
Presently, many commercially available cables also include a water-swellable tape. The tape is used to prevent the travel of water through the sheath system and into the core as well as its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, including a water-swellable powder which is trapped between two cellulosic tissues. Further included may be a polyester scrim which is used to provide tensile strength for the laminated tape. Although such a tape provides suitable water protection for the cable, it is relatively expensive and thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware.
Another factor that must be considered with respect to a waterblocking system for a cable is the bonding of the plastic cable jacket to an underlying metallic shield. Where such adhesion is important to the performance of the cable, care must be taken not to interpose a waterblocking member therebetween which would impair the desired adhesion.
As a solution to the foregoing problems, a waterblocking member in the form of a strip or a yarn which covers only an insubstantial portion of an inner periphery of the cable may be used. In this way, the strip or the yarn separates only an insubstantial portion of the jacket from other portions of the sheath system. Hence, if adhesion between the jacket and the other portions of the sheath system is desired, that adhesion is not compromised by the waterblocking member. Further, such a strip or yarn is less expensive than one which covers substantially an entire inner periphery of the cable.
Further, the prior art discloses that a waterblocking member may extend linearly or helically along the cable. In an optical fiber cable in which separate strength members extend linearly within the cable, the strip or yarn may be wrapped helically about a core tube along an outer surface of which extend the strength members. In an optical fiber cable in which the strength members extend helically about the cable core, the yarn or strip extends linearly or is wrapped in a helical direction opposite to that of the strength members and is disposed between the strength members and the core. See U.S. Pat. No. 4,815,813 which issued on Mar. 28, 1989 in the names of C. J. Arroyo, H. P. Debban, Jr., and W. J. Paucke.
In the last mentioned optical fiber cable, water may travel along a helically or linearly extending channel formed along each helically or linearly extending strength member. The water is intercepted at each point at which a waterblocking yarn or strip crosses a strength member. However, in metallic conductor cables, strength is provided by the metallic conductors themselves and by metallic shields of the sheath system. In those instances, any water is not channeled along helically or linearly extending paths such as along the helically or linearly extending strength members in optical fiber cables, but rather can travel along an annularly shaped channel between adjacent components of the cable.
Another problem relates to a cable which includes an inner jacket which may be used to cover a plastic core wrap material such as Mylar.RTM. plastic, for example. If a metallic shield is contiguous to the plastic core wrap material, the core wrap material may be flooded with an atactic material for waterblocking purposes. Here again such materials as atactic flooding compounds are not popular with craftspeople who at some future time may have to reenter the cable and be faced with housekeeping problems. On the other hand, if an inner jacket is interposed between the core wrap and the metallic shield, it becomes difficult to extrude a jacket having a uniform thickness over the flooding material. Furthermore, lumps could appear in the jacket, caused by uneven masses of the underlying flooding material.
Seemingly, the prior art does not disclose a cable which is provided with a system which prevents substantially the flow of water longitudinally along a cable sheath system which has no helically or linearly extending strength members and in which a plastic jacket is to be extruded directly over a relatively supple plastic core wrap. What is needed and what does not appear to be available in the marketplace is a cable waterblocking system which is relatively inexpensive and which does not add significantly to the diameter of the cable. Such a system should be one which is easily provided during the cable manufacturing process.